Gyromagnetic mode travelling-wave parametric amplifier and oscillator



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ATTORNEY United States Patent 3,120,646 GYROMAGNETIC MODETRAVELLING-WAVE PARAMETRIC AlVIPLIFIER AND OSCILLATOR Harold Seidel,Fanwood, N.J., assignor to Bell Telephone Laboratories, Incorporated,New York, N.Y., a corporation of New York Filed Oct. 25, 1961, Ser. No.147,641 11 Claims. (Cl. 330-46) This invention relates to theamplification and generation of high frequency electromagnetic waveenergy and more particularly to traveling wave, solid state parametricamplifiers and oscillators.

In applicants paper entitled Character of Waveguide Modes inGyromagnetic Media published in the Bell System Technical Journal,volume 36, March 1957, pages 409 to 426 and in a paper by applicant andR. C. Fletcher entitled Gyromagnetic Modes in Waveguide Partially Loadedwith Ferrite published in the Bell System Technical Journal, volume 38,November 1959, pages 1427 to 1456, the transmission of wave energy inisotropic and in anisotropic waveguides is considered. In particular, itis shown that there are an infinite number of modes capable ofpropagating in a waveguide loaded with gyromagnetic material no matterhow small the guide. Since these modes have no analog in waveguidesfilled with isotropic material, these modes have been characterized asgyromagnetic modes.

It is a characteristic of gyromagnetic modes that the wave energyassociated therewith tends to concentrate about a boundary or interfaceof the gyromagnetic material. It is also a characteristic ofgyromagnetic modes that the distribution of wave energy is virtuallyindependent of the nature of the waveguiding structure. This is incontrast to the normal Waveguide modes which propagate ingyromagnetic-loaded waveguides. Although the field pattern for thesenormal modes is distorted by virtue of the presence of the gyromagneticmaterial, the field pattern is nevertheless influenced to a considerableextent by the presence of the Waveguiding structure. By contrast, thedistribution of 'wave energy for the gyromagnetic modes is a function ofthe nature of the gyromagnetic interface to which the wave is bound.Thus, for example, at .a gyromagnetic-air boundry, the fielddistribution is a maximum at the plane of the interface and drops offexponentially as a function of the distance from the interface.

In applicants copending applications Serial No. 60,429, filed October 4,1960, now US. Patent 3,010,084, issued November 21, 1961 and Serial No.774,547, filed November 17, 1958, now U.S. Patent 3,010,086, issuedNovember 21, 1961, there are disclosed a new class of isolators whoseoperation is based upon the generation and propagation of thesegyromagnetic modes. It is now proposed, however, to utilize these modesin connection with parametric amplifiers and oscillators of reducedsize, simple construction and greater inherent stability.

Accordingly, it is the broad object of this invention to obtainparametric amplification of high frequency wave energy propagating inone of the gyromagnetic modes.

As is well known, the magnetization of a gyromagnetic material precessesunder the influence of a pumping magnetic field at a frequency 7",, anda steady polarizing magnetic field. If new there is superimposed uponsuch a system the magnetic field of a signal wave at a frequency f thesteady field in modulated so that the precessional frequency of themagnetization is varied. In the presence of the gyromagnetic materialthe signal wave mixes with the pumping wave to produce a component ofmagnetic field at the so-called idler frequency f, such that In a paperby P. K. Tien and H. Suhl entitled A Traveling-Wave FerromagneticAmplifier, published in the April 1958 Proceedings of the Institute ofRadio Engineers, pp. 700 to 706, it is further pointed out that in atraveling wave system an additional, preferred condition for parametricamplification is that the phase constant of the pumping wave 5;, and thephase constants of the signal [i and the idler 8 be related such that Itis, accordingly, a more specific object of this invention to obtainparametric amplification of signal wave energy by suitably relating thefrequency and phase constants of propagating gyromagnetic mode waves.

In accordance with the invention, guided wave energy propagating in anisotropic wave path in one of the normal propagating TE, TM or TEM modesis coupled from said normal mode to one of the gyromagnetic modes ofwave propagation. Specifically, gyromagnetic propagating modes areinduced in a gyromagnetic element at a signal frequency and at a pumpingfrequency higher than said signal frequency. By suitably selecting thegyromagnetic modes induced and the electrical properties of thegyromagnetic material, the preferred frequency and phase relationshipsare established for parametric amplification.

In a first embodiment of the invention a composite element consisting ofthree slabs of gyromagnetic material is disposed in a Waveguidingstructure whose internal dimensions are such that the loaded guide iscut-off for the normal waveguide modes over the frequency range ofinterest. This insures that the only propagating modes are thegyromagnetic modes. The three slabs are in the form of flat platesarranged with their broad surfaces contiguous and parallel to each otherand to the direction of the applied steady biasing field. The plates areenergized in a manner to induce the same order gyromagnetic mode at thepumping frequency and at the signal frequency.

In a second embodiment of the invention two distinctly differentgyromagnetic modes are induced in a single gyromagnetic element. One ofthe modes is derived from a TE mode wave whereas the other mode isderived from a TEM mode wave. Scattering techniques are used to coupleenergy from the TEM mode to one of the gyromagnetic modes.

It is an advantage of an amplifier constructed in accordance 'with theinvention that it has greater inherent stability and, consequently, doesnot require separate means for suppressing amplification in the reversedirection. This stability comes about by virtue of the fact that thegyromagnetic modes are nonreciprocal and, hence, the conditionsfavorable for amplification in one direction of propagation are notsatisfied for propagation in the reverse direction. A second advantageof an amplifier constructed in accordance with the invention resides inthe fact that amplification is produced uniformly and continuously alongthe gyromagnetic material rather than at discrete intervals.Furthermore, the gyromagnetic modes propagate with a propagationvelocity several orders of magnitude smaller than the normal waveguidemodes. As such they constitute a slow-wave structure of exceedinglysimple design which affords a high level of interaction between thepumping wave and the signal wave thereby resulting in a substantialamount of gain per unit field distribution about the interfaces of thecomposite gyromagnetic element used in the embodiment of the inventionshown in FIG. 1;

FIGS. 3 and 4 show, by way of illustration, the manher in which thephase constant of the gyromagnetic mode varies as a function of theangular frequency and further illustrates the manner in which anamplifier iri accordance with the invention can be graphically designed;

FIG. 5 is a second embodiment of the invention using a combination of TEand TEM mode waves to induce two different gyromagnetic mode waves ofdifferent order;

FIG. 6 given by way of explanation, illustrates the manner in which thephase constant of the gyromagnetic modes induced in the embodiment ofFIG. 5 vary as a function of the angular frequency;

FIG. 7 shows a modification of the embodiment of FIG. 5; and

FIG. 8 shows the cross section of an amplifier, in accordance with theinvention, adapted for use with the circular electric mode of wavepropagation.

Referring to FIG. 1, there is shown a first embodiment of a parametricamplifier in accordance with the principles of the invention comprisingfirst and second longitudinally spaced sections 10 and 11 of boundedelectrical transmission line for guiding electromagnetic wave energy.The sections are coaxially aligned along a common longitudinal axis xxand can be of the metallic shield type having a rectangularcross-section whose Wide dimension is at least one-half wavelength ofthe wave energy to be propagated therethrough, and whose narrowdimension is typically one-half of the wide dimension. S0 proportioned,waveguides 10 and 1 1 are supportive of one of the normal TE or TMwaveguide modes including, at least, the dominant mode, known in the artas the TE mode, in which the electric lines of force extend from thebottom to the top of the waveguide, perpendicular to the wide guidewalls and in which the intensity of the electric field variessinusoidally along the wide dimension, reaching a maximum at the centerof the guide and being substantially Zero at the edges.

Located between guides 10 and 11 is a third section of reduced widthwaveguide 12 whose effective transverse cross-sectional dimensions (whenloaded with gyromagnetic material) are to be less than one-half of thefreespace wavelength of the wave energy to be propagated therethrough.Thus, though characterized as a waveguide, section 12 is, in fact,proportioned to be cut-off for the normal waveguide modes since, to theextent that such modes do exist in guide 12, they represent a loss.

In the embodiment of FIG. 1, waveguide 12 is illustrated as having arectangular cross-section. The crosssectional geometry of guide '12 can,however, be oval or circular without in any way affecting the operationof the device. Advantageously, however, the transverse dimensions areless than one-half the free-space wavelength of the energy to bepropagated therethrough, as indicated hereinbefore.

Located Within section =12 is a composite element 13 of active materialcomprising the three slab-like elements 1, 2 and 3 each of a differentgyromagnetic material. The term gyromagnetic material is employed herein its accepted sense as designating the class of magnetic polarizablematerials having unpaired spin systems involving portions of the atomsthereof that are capable of being aligned by an external magneticpolarizing field and which exhibit a significant precessional motion ata frequency within the range contemplated by the invention under thecombined influence of said polarizing field and an orthogonally directedvarying magnetic field component. This precessional motion ischaracterized as having an angular momentum and a magnetic moment.Typical of such materials are ionized gases, paramagnetic materials andferromagnetic materials, the latter including the spinels such asmagnesium aluminum ferrite, aluminum zinc ferrite and the garnet-likematerials such as yttrium iron garnet. In the embodiment shown in FIG.1, element 13 is made of ferrite materials. Accordingly, in thediscussion to follow the terms ferrite, ferrite-air, ferrite-metal, etcetera, will be used to describe various aspects of the invention.However, it is to be understood that other gyromagnetic materials can beused equally as well and that the use of ferrite is merely intended tobe illustrative.

The elements 1, 2 and 3 are aligned with their broad surfaces parallelto each other and to the narrow walls of guides 10 and 111. The adjacentbroad surfaces of elements 1 and 2 are placed in contact defining afirst interface 12, and the adjacent broad surfaces of elements 2 and 3are placed in contact defining a second interface 2-3.

Composite element 13 is preferably symmetrically located within guide 12although its precise location therein is not critical. Element 13 caneither completely fill guide 1 2 or it can be spaced from any of thevarious guide walls. In the embodiment of FIG. 1, element 13 is shown incontact with the top and bottom walls but spaced away from the sidewalls of guide 12. To facilitate coupling between the waveguide mode andthe gyromagnetic mode, the composite element 13 advantageousiy extendsinto guides 10 and 11 and is preferably tapered at both ends.

A static magnetic polarizing field H is applied in a direction parallelto the broad surfaces of elements 1, 2 and 3. The polarizing field canbe supplied by any suitable means (not shown) such as an electricsolenoid, a permanent magnetic structure or, in some instances, theelements 1, 2 and 3 can be permanently magnetized.

In operation, wave energy at a pumping frequency f indicated by an arrow14, is applied to waveguide 10 from a source of wave energy (not shown).Simultaneously, signal wave energy at a frequency i indicated by anarrow 15, is applied to waveguide 10 from a second source of wave energy(not shown). Wave energy at both these frequencies propagates alongguide 10 in one of the normal TE waveguide modes. Upon reaching guide12, however, these normal TE waveguide modes cannot propagate due to thefact that the ferrite-loaded guide 12 is cut-off at both frequencies andi However, as was pointed out in the above-cited paper by H. Seidel andR. C. Fletcher, there are an infinite number of modes capable ofpropagating in a waveguide partially filled with gyromagnetic materialno matter how small the guide. These modes are called gyromagnetic modessince they have no analog in waveguides filled with isotropic material.I

Typically, the gyromagnetic modes propagate as boundary waves in whichthe energy concentrates at one of the interfaces of the gyromagneticmaterial, falling off exponentially as a function of distance from theinterface. The various gyromagnetic modes differ from each other in thespatial distribution of the magnetization vectors within thegyromagnetic material and are related to the inducing mode and themanner of coupling between the inducing mode and the gyromagnetic mode.In the embodiment of FIG. 1 the primary mode induced in the gyromagneticmaterial is the lowest order, 111:0, gyromagnetic mode.

FIG. 2 is a cross'sectional view showing the three gyromagnetic elements1, 2 and 3 and guide 12. It will be noted that there are fourinterfaces. The first is the ferrite-air interface associated withelement 1; the second is the ferrite-ferrite-interface 1-2; the third isthe ferrite-ferrite interface 2-3; and the fourth is the ferriteairinterface associated with element 3. Each of these is capable ofsupporting gyromagnetic modes.

In accordance with the invention, however, the electrical and physicalproperties of composite element 13 are selected so that the frequenciesof interest, the wave energy is concentrated at the -1-2 and 23ferrite-ferrite interfaces. In particular, if the pumping wave energy isconcentrated at one interface and the signal wave energy at the otherand if, in addition, element 2 is thin, there is a substantial region ofover-lap for the two waves and a resulting strong region of interactionof the waves and the gyromagnetic material. This is illustrated in FIG.2 in which the two curves 20' and 21 show the distribution of waveenergy at interfaces L2 and 2-3 for the pumping frequency f and thesignal frequency i By making the transverse dimension 1 of element 2small, there is considerable interaction between the waves and element2. Furthermore, since the velocity of propagation of the gyromagneticmodes is one or more orders of magnitude less than that of the normalWaveguide mode, this interaction persists for a relatively long time perunit length of gyro-magnetic material.

In the above-mentioned publication by P. K. Tien and H. Suhl, theoptimum conditions for parametric amplification in a traveling waveconfiguration are given as where f B f [3 and f 3 are the frequenciesand phase constants of the pumping wave, signal wave and the idler wave,respectively. -By virtue of the interactions of the pumping 'wave andthe signal wave in the gyro magnetic material, it is apparent that thefirst condition is automatically satisfied. Condition 2, though notessential, defines the condition of maximum gain. Tien and Suhl showthat to the extent that Equation 2 is not satisfied, the gain isreduced. Accordingly, condition 2 is the preferred condition.

Accordingly, the preferred design of a parametric amplifier inaccordance with the invention involves a selection of suitablegyromagnetic material whose electrical properties are such as to make itpossible to satisfy the frequency and phase constant requirements setforth in Equations 1 and 2. An amplifier in accordance with theinvention can be readily designed graphically from a plot of thefrequency-phase constant curve for each ferrite-to-ferrite interface.Typically, the phase constant increases as a function of frequency andapproaches infinity at a critical frequency m This occurs at the 12interface for the m== gyromagne-tic mode at for which U+=% 4TMZ+ 41rM 4and w gering-41114 5) where:

41rM and 41rM are the saturation magnetizations for gyromagnetic slabs'1 and 2, respectively,

'y is the ratio of magnetic moment to angular moment-um for an electron,generally equal to 2.8 mc. per oersted, and

H is the steady biasing field.

Knowing the frequency of the signal to be amplified, 41rM 41rM and H areselected such that w the critical frequency for interface 1-2, issomewhat higher than the signal frequency.

Using the same values already given for H and 41rM a similar calculationis made for the 2-3 interface. Since the pumping frequency is equal totwice the average of the signal frequency and the idler frequency, 41rMis selected such that w is slightly less than twice The saturationmagnetization of ferrite materials can be varied in many ways as, forinstance, by varying the ratio lOf magnetic to nonmagnetic materials ineither the divalent or trivalent sites or by varying the density of theferrite material. For -a discussion of ferrites and, in particular, thesaturation magnetization of ferrite see Ferrites by J. Smit and P. I.Wijn, pages 147 to published in 1959 by J. Wiley & Sons.

FIG. 3 is a graph whose abscissa is angular frequency .w, and Whoseordinate is phase constant 5. ,The two critical frequencies w and w havebeen plotted and a portion of the fiw curves for the two interfaces havealso been plotted. The latter are computed for large values of ,6 sincethe region of large phase constants define a preferred range ofoperation in that they imply a slower traveling wave and hence a longerinteraction time for the pumping and signal waves in the gyromagneticmaterial.

To arrive at an operating point, the (1-2) interface curve 39 isreplotted by doubling the frequency and phase constant values at pointsalong curve 30 to obtain a second 1-2 interface curve 32 shown dotted.For example, a point 1 on curve 30 having an angular frequency w and aphase constant 5 is plotted as point 2 on curve 32 at a frequency 2w anda phase constant 2 8. Curve 32 intersects the 2-3 interface curve 31 ata point P. Where curve 32 intersects curve 31 defines a point on curve31 (o and a point on curve 30 (m fl This point satisfies the conditionsfor parametric amplification in the degenerate mode. That is Zw5=w andfls flp For operation in the nondegenerate mode, we proceed as before byidentifying two critical frequencies and plotting a portion of eachinterface curve as shown in FIG. 4. However, curve 42 which is derivedfrom curve 40 is obtained in a slightly different manner than curce 32in FIG. 3. Curve 42 is obtained by selecting two frequencies w-A andw-I-A which represent the idler and signal frequencies where 2A is thedesired separation between said frequencies. For w-A there is acorresponding phase constant [3* and for w-i-A there is a correspondingphase constant 5+. Curve 42 is a plot of (B-+/3+) as a function of[(wA)+(w+A)]. The intersection Q of curve 42 with the 23 interface curvedefines the operating point for parametric amplification in thenondegenerate mode. That is, it defines a point for which the sum of thesignal and idler frequencies equals the pumping frequency and for whichthe sum of the phase constant of the signal and idler equals the phaseconstant of the pumping wave.

For an amplifier to be operated at a signal frequency of about 500megacycles per second, saturation magnetizations of 357, 760 and 1430gauss for elements 1, 2 and 3 would be typical. The critical frequencyat the two interfaceswould be 563 megacycles per second and 938mepacycles per second.

2 Another consideration in the design of an amplifier in accordance withthe embodiment of the invention shown in FIG. 1 is the thickness t ofthe center slab 2. Preferably the slab is very thin so that there is amaximum overlapping of the field distribution at the two interfaces.However, if the slab is too thin the interfaces are no longer clearlydefined electrically and the boundary waves are not properlyestablished. Accordingly, a compromising thickness is recommended forwhich the field has decreased to approximately one-third to one-tenth ofits maximum intensity. Since the field decreases exponentially as afunction of distance from the interface and is related to the phaseconstant p of the interface mode, the preferred thickness is definedsuch that 9 -3 to g N lfil 161 where {3 is the phase constant for thelowest frequency Wave (i.e., the idler or signal frequency).

In the embodiment of FIG. 1 described above, both the signal and thepumping wave propagated in waveguides 10 and 11 in one of the normal TEwaveguide modes and the lowest order, m=0, gyromagnetie mode was inducedin the gyromagnetic material. However, other types of incident modes andhigher order gyromagnetic modes can be used for either the signal or thepumping wave or for both. This permits a modification in structure andan alternate embodiment of the invention as shown in FIG. 5.

This second embodiment of the invention comprises a pair oflongitudinally spaced rectangular waveguides 50 and 51 proportioned tosupport the pumping wave in a TE waveguide mode. Interposed betweenguides 50 and 51 is a third waveguide 52 of reduced width within whichthere is located an element 53 of magnetically biased gyromagneticmaterial. In the illustrative embodiment of FIG. 5, element 53 is in theform of a thin slab of ferrite material which extends longitudinallywithin guide 52 and is transversely displaced with respect to the guideaxis so that one of the broad surfaces of element 53 is in contact withone of the vertical walls of guide 52 defining a ferrite-metalinterface. The opposite broad surface of element 53 is exposed towhatever dielectric material fills guide 52. In the embodiment of FIG.the dielectric filling is assumed to be air. Accordingly, this oppositesurface defines a ferrite-air interface.

Ferrite element 53 preferably extends into guides 50 and 51 a slightdistance and is tapered in a manner to minimize reflections and tocouple between the waveguide mode and the gyromagnetic mode. It will benoted, that in contrast to the composite gyromagnetic element 13 used inthe embodiment of FIG. 1, element 53 is composed of only one type ofgyromagnetic material.

The signal wave in the embodiment of FIG. 5 is propagated in the TEMmode by means of a two conductor transmission line which comprises asthe outer conduc tor the waveguides 50, 51 and 52 and an inner conductor54 which is shown as a thin rectangular conductive member extendinglongitudinally through the three guides 50, 51 and 52.

Because coaxial transmission lines do not cut-off in the same sense asdo waveguides and because the propagation properties of the gyromagneticmodes are substantially different than the TEM mode, there is littletendency for the signal wave to, couple from the TEM mode to any of thegyromagnetic modes. Accordingly, special means are provided to effectsuch coupling. In my copending application Serial No. 774,547, referredto above, now United States Patent 3,010,086 issued November 21, 1961, anumber of scattering techniques are shown to accomplish this coupling.One of the methods used and described in some detail in said copendingapplication, comprises placing a number of discontinuities along thewave path in the form of indentations 55 along the inner conductor 54immediately adjacent to the gyromagnetic material 53. Thediscontinuities effectively interrupt the longitudinal symmetry of thewave path creating higher order space harmonics. In the presence of thegyromagnetic material, wave energy is coupled between the modified TEMmode wave and one or more of the higher order gyromagnetic mode waves.

Because the two gyromagnetic modes induced in the gyromagnetic materialin the embodiment of FIG. 5 are derived from two distinctly differenttypes of incident modes (the TB mode and T EM mode) and different cou- 8pling means, the gyromagnetic modes themselves are also different. Assuch, each of the gyromagnetic modes can be separately selected tosatisfy the preferred frequency and phase velocity conditions as setforth above in Equations 1 and 2 and for this reason only one type ofgyromagnetic material is needed.

In FIG. 2 of the publication by applicant and R. C. Fletcher citedabove, a plot of the variation of the phase constant as a function ofthe steady biasing magnetic field is shown for various propagatinggyromagnetic modes. In FIG. 6 a slightly modified version of this plotis given showing the variation of phase constant as a function ofangular frequency for the m=0 mode and for three arbitrarily selectedhigher order modes m=n "1:11 and 121211 The m=0 mode wave propagatesbetween frequency (.0 and a where It will be noted that in this intervalthe m=0 mode is capable of propagating as either a backward wave(negative 15) at the ferrite-air interface or as a forward wave(positive 5) at the ferrite-metal interface.

The nz=n mode waves propagate between frequencies a and 00 where In thisinterval, the M1 11 modes also propagate either as a backward wave(negative 5) at the ferrite-air interface or as a forward wave (positiveB) at the ferrite-metal interface.

To ascertain an optimum operating point, a graphical construction of thetype outlined above is made. Because of the plurality of higher ordermodes and the two modes of propagation that are possible for all themodes, various combinations of positive and negative propagating wavesare possible. One combination of modes includes a ferrite-metal mode forthe signal and a ferrite-metal mode for the pumping wave.

Because the propagation constant at any frequency increases with themode order, operation with a higher order mode is to be preferred. Thedistribution of modes induced is a function of the discontinuities 55along conductive member 54. In general, the larger the number ofdiscontinuities per unit length and the more abrupt the discontinuities,the higher the mode order of the induced gyromagnetic mode.

Some improvement in the conversion efficiency from the TEM mode to theferrite-metal gyromagnetic mode can be effected by reducing the distancebetween the discontinuities and the ferrite-metal interface. This can beaccomplished by cutting a longitudinal slot along the ferrite andinserting the conductive member in the slot thus cut. This isillustrated in FIG. 7 which shows a portion of reduced width waveguide70, ferrite element 72 including a slot 73 and the inner conductor 71located within said slot 73. This has the effect of placing theindentations 74 closer to the ferrite-metal interface.

Additional improvements can be realized in the several embodiments ofthe invention described above by reducing the height of the gyromagneticmaterial. This has the effect of increasing the phase constant of thegyromagnetic modes (reducing the velocity of propagation) therebyincreasing the effective interaction time and thus increasing the gainper unit length of gyromagnetic material.

Many of the currently popular solid-state travelingwave amplifiers, suchas the three level maser described by R. W. De Grasse, J. J. Kostelnickand H. E. -D. Scovil in an article entitled The Dual Channel 2390-mcTraveling-Wave Maser, published in the July 1961 Bell System TechnicalJournal, pages 1117 to 1127, are reciprocal in and 9 their action and,consequently, separate and distinct means must be provided in order tosuppress amplification in the reverse direction and thus insure stableoperation of the amplifier. Such special precautions are unnecessary,however, for amplifiers constructed and operated in accordance with theteachings of the instant invention since they are inherently stable inthat their action is nonreciprocal. For wave energy propagating in thereverse direction, the situation is radically altered and favorableconditions for parametric amplification are not satisfied. To thecontrary, incident wave energy in the reverse direction is eitherdissipated by absorption within the gyromagnetic material or propagatesthrough :without interacting in the manner to produce appreciableamplification. Hence, amplifiers of the type described herein areinherently much more stable than the bilateral amplifiers of the typedescribed by De Grasse et al.

In the various embodiments described, one or more of the incident modeswere characterized as TE mode wave propagating in a rectangularwaveguide. This was in no way intended to limit the invention to anyparticular mode wave. To the contrary, other modes, such as TE modes incircular waveguides and TM modes in rectangular and circular waveguidescan be used. For example, FIG. 8 shows a cross-section of an embodimentof the invention similar to the embodiment shown in FIG. 1 adapted tooperate in conjunction with the circular electric mode of Wave energyand comprises a circular waveguide 80 and three concentric ferritecylinders 81, 8'2 and 83 circumferentially biased by means of steadymagnetic biasing field H induced in any convenient manner well known inthe art. Adjacent surfaces of the ferrite cylinders are in contact toform a pair of ferrite-ferrite interfaces. The ferrite-loaded guidewhich is cut-off for the circular electric mode at the signal andpumping frequencies is typically placed between sections of circularwaveguide supportive of the circular electric mode at the frequencies ofinterest and in all respects operates in the manner described above inconnection with the embodiment shown in FIG. 1.

The technique of cutting off the waveguide is a convenient, although notnecessary, means of coupling between the incident mode and thegyromagnetic mode. Other techniques (such as, for example, thescattering technique used in connection with the embodiment of FIG. canalternatively be used to couple between an incident mode and agyromagnetic mode. The efficiency of the amplifier, however, varies as afunction of the efficiency of the mode conversion technique employed. Itis, of course, preferred that all of the incident wave energy beconverted into the desired gyromagnetic mode. To the extent that this isnot accomplished, the unconverted or improperly converted wave energy iseither not amplified or, in some instances, is attenuated and representsinstead a loss to the system.

While the various embodiments described above have been characterized asamplifiers, it is known that by increasing the amplitude of the pumpingwave about a critical threshold level for the system, a parametricamplifier can be made to oscillate. This is pointed out by H. Suhl inhis article Proposal for a Ferromagnetic Amplifier in the MicrowaveRegion, published in The Physical Review, vol. 106, April 15, 1957.Thus, any of the illustrative embodiments of the invention describedherein can be used as an oscillator, the only difference being that nosignal wave is applied to the device.

It is, accordingly, understood that the above-described arrangements areillustrative of but a small number of the many possible specificembodiments which can represent applications of the principles of theinvention. Numerous and varied other arrangements can readily be devisedin accordance with these principles by those skilled in the art withoutdeparting from the spirit and scope of the invention.

What is claimed is:

1. A parametric amplifier comprising a slab of magnetically polarizedgyromagnetic material having a pair of spaced planar interfaces;

means for inducing a first gyromagnetic propagating mode having a phaseconstant 3 at one of said interfaces at a frequency A; means forinducing a second gyromagnetic propagating rnode having a phase constant25 at the other of said 10 interfaces at a frequency 2f said interfacesbeing spaced a distance 1 given approximately by and means forextracting amplifier wave energy from said amplifier at said frequency f2. A parametric amplifier comprising first and second waveguiding pathscapable of supporting energy in one of the normal modes of wavepropagation at a signal frequency f, and at a pumping frequency of fhigher than said signal frequency;

a third waveguiding path proportioned to be cut-off for said normalwaveguide modes disposed between and electromagnetically coupled to saidfirst and second waveguiding paths; said third waveguiding pathincluding three slabs of dissimilar gyromagnetic materials defining apair of spaced planar interfaces;

means for magnetically biasing said slabs; one of said interfaces beingcapable of propagating said pumping frequency wave in a gyromagneticmode of wave propagation with a phase constant ti the other of saidinterfaces being capable of propagating said signal frequency wave in agyromagnetic mode of wave propagation with a phase constant [3 saidother interface being further capable of propagating a third signal in agyromagnetic mode of wave propagation;

said third signal having a frequency (f f and a phase constantsubstantially equal to the difference between B1: and 1 s; and saidinterfaces being spaced in distant t given by where B is the phaseconstant of the lowest frequency wave.

3. A parametric amplifier comprising first and second longitudinallyspaced rectangular waveguides supportive of wave energy at a signalfrequency i and at a pumping frequency f higher than said signalfrequency in a normal waveguide mode of propagation;

a third section of waveguide proportioned to be cut-off at said signalfrequency and at said pumping frequency for said normal mode of Wavepropagation disposed between said first and second waveguides;

means for propagating said wave energy through said third waveguide in agyromagnetic mode of wave propagation comprising three slabs ofmagnetically biased gyromagnetic material having different saturationmagnetizations;

said slabs forming a composite structure with one of said slabs locatedbet-ween and contiguous with the other two slabs defining a pair ofgyromagnetictogyromagnetic interfaces;

one of said interfaces being capable of supporting wave energy in agyromagnetic mode of wave propagation at said signal frequency i and atan idler frequency f, with phase constants B and 8 respectively;

and the other of said interfaces being capable of supporting wave energyin a gyromagnetic mode of wave propagation at said pumping frequency fwhere f =f +f with a phase constant fl +fig said interfaces beingseparated a distance t given by 2.3 3.4 t 1n ml where n is the phaseconstant of the lowest frequency signal.

4. The combination to claim 3 wherein the saturation magnetization ofsaid one slab is intermediate the saturation magnetization of the othertwo slabs.

5. A parametric amplifier for amplifying signal frequency wave energycomprising a signal source having a frequency f for supplying signalwave energy to said amplifier;

a pumping source having a frequency f higher than 1 for supplyingpumping energy to said amplifier;

a pair of longitudinally spaced waveguides of rectangular cross sectionsupportive of said signal and said pumping wave energy;

each of said guides having a pair of narrow and a pair of wideconductive walls;

said guides being aligned along a common longitudinal axis with the widewalls of each parallel to the wide walls of the other;

a ferrite-loaded section of wave path proportioned to be cut-off at saidsignal and said pumping frequencies comprising a third section ofconductively bounded waveguide connecting said first guide and saidsecond guide;

three slab-like elements of ferrite material having different saturationmagnetizations located within said third section;

each of said elements having a pair of parallel broad surfaces whichextend throughout the length of said third section in a directionparallel to the narrow walls of said pair of guides with a broad surfaceof each element parallel to and contiguous with a broad surface of thenext adjacent element thereby defining a pair of ferrite-to-ferriteinterfaces;

one of said interfaces being supportive of wave energy in a gyromagneticmode of wave propagation at the frequency of said signal source and at asecond frequency equal to the difference between the pumping frequencyand the signal frequency;

the wave energy at said signal frequency and at said second frequencyhaving a propagation constant 13 and 6 respectively;

the other of said interfaces being supportive of wave energy in agyromagnetic mode of wave propagation at said pumping frequency andhaving a phase constant equal to the sum of [3 and 5 said interfacebeing separated by a distance 1 given by where [3 is the phase constantof the lowest frequency wave;

means for coupling wave energy between said pair of guides and saidthird section comprising a tapered extension of said elements into saidsaid pair of Waveguides;

and means for magnetically biasing said elements in a direction parallelto their broad surfaces.

6. A parametric amplifier comprising a two conductor transmission linehaving an inner conductive member and an outer conductive membersurrounding said inner conductive member;

said line being capable of supporting wave energy at a signal frequencyin a TEM mode of wave propagation;

said outer conductive member capable of supporting pumping wave energyat a frequency f higher than said signal frequency in one of the normalwaveguide modes of propagation;

a portion of said line containing a slab of magnetically biasedgyromagnetic material capable of sup- 12 porting gyromagnetic modes ofwave propagation at said signal frequency and at said pumping frequencywith phase constants B and 8 respectively;

said material being further capable of supporting wave energy at a thirdfrequency equal to f -f with a phase constant approximately equal to [t-fi said material being asymmetrically disposed between said innermember and said outer member; said outer member having reducedcross-sectional dimensions over said portion of line whereby said outermember is cut-off at said pumping frequency for propagation in saidnormal waveguide mode;

and means for coupling said signal frequency wave energy from said TEMmode to one of said gyrornagnetic modes of wave propagation comprising aplurality of electrical discontinuities longitudinally distributed alongsaid portion of line.

7. The amplifier according to claim 6 wherein all of said gyromagneticmodes propagate along the same boundary of said gyromagnetic material.

8. The amplifier according to claim 6 wherein said gyromagnetic modespropagate along different boundaries of said gyrornagnetic material andwherein said boundaries are separated by a distance 1 given by where Bis the phase constant of the lowest frequency wave.

9. A parametric amplifier comprising first and second sections oflongitudinally displaced circular waveguides supportive ofelectromagnetic wave energy in the circular electric mode of wavepropagation at a signal frequency i and at a pumping frequency f higherthan said signal frequency;

a length of reduced diameter circular Waveguide proportioned to becut-off for said circular electric mode of wave propagation at saidsignal and at said pumping frequency located between said sections ofwaveguide;

and means for propagating said wave energy through said length ofcut-off waveguide comprising three hollow coaxial circular cylinders ofcircumfercntially magnetically biased gyromagnetic material havingdifferent saturation magnetization;

said cylinders extending coaxially along said length from said firstwaveguide to said second waveguide with a surface of each of saidcylinders contiguous to at least one surface of another of saidcylinders to define a pair of gyromagnetic-to-gyromagnetic boundaries;

one of said boundaries being supportive of wave energy in thegyromagnetic mode of wave propagation at said signal frequency with aphase constant 5 and at an idler signal at a frequency f -f with a phaseconstant 5;;

the other of said boundaries being supportive of wave energy in agyromagnetic mode of wave propagation at the pumping frequency with aphase constant substantially equal to 19 and said boundaries beingspaced from each other a distance t to means for applying pumping waveenergy to said os- 3,1 13 cillator at a level greater than the thresholdlevel for said oscillator; means for inducing said gyrornagneticpropagating modes in said material at said pumping frequency; and meansfor extracting energy from said oscillator at at least one of said lowerfrequencies.

11. The oscillator according to claim 10 wherein said modes propagatealong different boundaries and wherein the distance t between boundariesis given by 2.3 3.4 I51 t0 Isl where B is the phase constant of thelowest frequency wave.

References Oited in the file of this patent UNITED STATES PATENTSSouthworth Jan. 13, 1959 Seidel Nov. 21, 1961 Kostelnick Jan. 16, 1962OTHER REFERENCES Seicl-el et 111.: Bell System Technical Journal, Novem-10 her 1959, pages 14271456.

1. A PARAMETRIC AMPLIFIER COMPRISING A SLAB OF MAGNETICALLY POLARIZEDGYROMAGNETIC MATERIAL HAVING A PAIR OF SPACED PLANAR INTERFACES; MEANSFOR INDUCING A FIRST GYROMAGNETIC PROPAGATING MODE HAVING A PHASECONSTANT B1 AT ONE OF SAID INTERFACES AT A FREQUENCY F1; MEANS FORINDUCING A SECOND GYROMAGNETIC PROPAGATING MODE HAVING A PHASE CONSTANT2B1 AT THE OTHER OF SAID INTERFACES AT A FREQUENCY 2F1;